SURFICIAL GEOLOGIC MAPPING ALONG THE SOUTH PLATTE RIVER CORRIDOR, EASTERN COLORADO, AND WHAT PRELIMINARY RESULTS IMPLY ABOUT RIVER RESPONSE DURING THE LATE PLEISTOCENE–HOLOCENE TRANSITION

The South Platte River flows from its headwaters in the Colorado Rocky Mountains across the semiarid, short-grass prairie of eastern Colorado, a region highly susceptible to drought. Part of the High Plains Ecoregion, the area is higher and drier than areas to the east, with a mean annual precipitation of about 30–50 cm and a mesic temperature regime. In this type of semiarid, drought-prone environment, geomorphic systems tend to be highly sensitive to climate change, and surficial deposits can provide a record of system response to that change. Past droughts have been severe enough to mobilize dune sand and produce significant deposits of windblown silt. In addition, alpine glaciations have affected the headwaters region of the river. New surficial geologic mapping, funded by the National Cooperative Geologic Mapping Program of the U.S. Geological Survey, has been initiated along a portion of the South Platte River corridor in eastern Colorado between Masters and Fort Morgan to better understand the river’s history and its relation to eolian deposits in the region. Along this stretch of river, late Pleistocene side-stream alluvium of Kiowa and Bijou Creeks forms a broad, low-relief fan that overlies and presumably interfingers with late Pleistocene main-stream alluvium of the South Platte River. Field observations of probable lake or pond sediments within the main stream alluvium upstream suggest that high sediment influx from these tributaries may have temporarily dammed the South Platte River at times during the late Pleistocene. Preliminary findings of mapping and geochronologic research (OSL and ¹⁴C dating) indicate that the surface associated with late Pleistocene side-stream alluvium could have been abandoned shortly after about 11.8 ka, around the time of the late Pleistocene–Holocene transition, and that the episode of river incision resulting in formation of this terrace surface may have been of brief duration; dates providing age estimates for side stream alluvium making up the next lower fill terrace suggest that the incision (about 15 m) could have occurred within the span of a couple hundred years and been followed by a period of renewed aggradation.